1. Field of the Invention
The present invention relates to an internal combustion engine with a supercharger and, more particularly, to a supercharged internal combustion engine having two or more intake valves disposed for one cylinder and a supercharger disposed in an intake passage.
2. Description of the Related Art
There are known internal combustion engines having a supercharger disposed in an intake passage with the attempt to improve filling efficiency and increase torque of the engine. The provision of the internal combustion engine with the supercharger in order to increase filling efficiency, however, suffers from the disadvantages that knocking is likely to occur in a low-rotation and high-load region and temperature of exhaust gases is likely to rise in a high-rotation and high-load region. This may adversely affect improvements in filling efficiency.
As technology of preventing supercharged engines from causing knocking, for example, Japanese Patent Unexamined Publication (Kokai) No. 2-119,620 discloses supercharged engines in which the timing of closing an intake valve is retarded or an overlap period of time during which the period of time in which the intake valve is opened overlaps with the period of time in which an exhaust valve is opened is prolonged. When the timing of closing the intake valve is retarded to a great extent after the bottom dead center (BDC), the effective compression ratio becomes reduced and an elevation of temperature is suppressed due to compression work, thereby improving anti-knocking performance. On the other hand, when the overlap period of time is prolonged, the anti-knocking ability can be improved by enhancing the scavenging ability and thereby reducing residual exhaust gases.
Further, for example, Japanese Patent Unexamined Publication (Kokai) No. 3-23,327 proposes supercharged engines so arranged as to make an air-fuel ratio lean in a supercharging region, in order to suppress temperature of exhaust gases from elevating.
In addition to the technology as disclosed in those references as described hereinabove, the knocking can be suppressed by making a cylinder bore size small shortening a distance in which flame expands or propagates or by retarding a timing of ignition. Further, it is desired to provide each cylinder with two or more intake valves, enlarging an opening area of the intake valve in order to increase an output of the engine.
It can be noted herein that various factors including a cylinder bore size, a timing of ignition, a compression ratio of the engine, a pressure ratio of a supercharger, and the like are associated with each other in order to prevent knocking and increase an output of the engine; however, for supercharged engines of conventional type, the relations of those factors with the anti-knocking performance and an increase of the engine output is not considered to a sufficient extent and there are still room for improvement in anti-knocking performance and the increase of the engine output.
Further, as the technique of suppressing the temperature of exhaust gases from elevating, it is known that the air-fuel ratio is made lean, the air-fuel ratio is made richer than required for output, and a compression ratio is made high. However, such a lean air-fuel ratio is disadvantageous in terms of output of the engine; such a richer air-fuel ratio is disadvantageous in terms of fuel economy, and such a high compression ratio is disadvantageous in terms of the anti-knocking performance.
In addition, Japanese Patent Unexamined Publication No. 60-237,153 discloses a supercharged engine in which exhaust gases are recirculated (EGR) in the supercharging region; however, this device for recirculating exhaust gases is arranged to recirculate the exhaust gases (EGR) with the attempt to reduce NOx. No attention is paid to anti-knocking ability and suppression of an elevation of exhaust gases.